Variable frequency power supplies are generally driven by a variable speed input. Such power supplies may be found, for example, on aircraft which have generators powered by turbine engines that operate over a wide speed range. The frequency of the power provided varies with the operating speed of the turbine. Some loads function adequately with variable frequency power; for other loads, power with controlled frequency is necessary or preferable.
Various methods are known for producing power with controlled or constant frequency from a variable speed generator. In such systems, variable frequency power is rectified and then input to an inverter which produces an output the frequency of which can be controlled. FIG. 1 illustrates a conventional system for converting variable frequency power to power with a controlled frequency. This system includes a variable frequency generator 200 connected to a power source such as turbine engine 201, an AC bus 202, a pair of autotransformers 204 connected to the AC bus 202, a pair of rectifiers 206 each having an input receiving multiphase power from transformers 204 and an output connected to a DC bus 208. A plurality of load circuits 210 are connected to DC bus 208. Each load circuit 210 includes an inverter 212, a DC contactor 214 isolating the inverter 212 from the DC bus, and an AC output 218. These load circuits often have low impedance, and therefore resonance between inverters can develop. It is therefore generally desirable to provide separation diodes 216 in the load circuits to isolate the load circuits from one another and minimize resonance.
Such systems work reasonably well for their intended purpose. However, DC contactors tend to be large and expensive and may fail before other elements of the circuit in which they are used. Moreover, the need for separation diodes adds to the complexity and expense of the such systems. It is therefore desirable to provide a method and system for providing power with controlled frequency from a variable frequency power source that reduces the need for DC contactors and separation diodes.